1. Field of the Invention
The present invention relates generally to solid state radiation detectors and, more particularly, to coaxial germanium gamma ray detectors. The present invention also relates to methods therefor.
2. Background of the Prior Art
The prior art has long recognized the utility of solid state radiation detectors; the most notable efforts in this area resulting in various techniques for the fabrication of P-I-N semiconductors. Fundamentally, these P-I-N structures provide an intermediate region of intrinsic or impurity-compensated semiconductor material sandwiched between P and N layers. Upon application of a reverse bias, depletion of electrical charge carriers within the intrinsic or impurity-compensated region yields an active zone for the detection of charged particles and x-ray and gamma ray photons. The event of impingement of such radiation is manifested by an ionization of the material within the intrinsic zone and the concomitant sweeping away and collection of resultant charges. By proper association of the detector with appropriate measuring equipment, such as, for example, pulse height analyzers, information regarding both the number of events and the distribution of energies is achievable.
The aforesaid ionization accompanying the event varies significantly with respect to the source of ionizing radiation. For example, many detectors which are suitable for interpreting charged particle radiation, such as alpha particle events, are wholly unsuitable for analysis of gamma and x-ray photons, which require much larger detectors with exceedingly lower noise characteristics.
The detection of x-ray or gamma ray photons is further complicated by the fact that the radiation may be absorbed or attenuated by various, diverse mechanisms. The most notable photon-electron interactions which give rise to indications of such events in radiation detectors are (1) ejection of photo-electrons, (2) Compton scattering, and (3) pair production or (4) pair annihilation. Additionally, bremsstrahlung radiation may affect variations in output signals from solid state detectors. Of lesser consequence to absorption and/or attenuation of an incident beam are (1) fluorescence, coherent scattering by (2) electrons and (3) atoms, and (4) Compton recoil electrons. For a more complete exposition regarding these interactions and the manner in which solid state radiation detectors respond thereto, see U.S. Pat. No. 3,524,985 to Sayres and the text Elementary Modern Physics, 2nd. ed., Allyn and Bacon, Boston, 1968.
Not all of the aforementioned mechanism account for the creation of charge carriers in the detector material. Some of the foregoing interactions will merely decrease the overall energy of the beam without yielding a charge carrier. Yet others will account for the ejection of an electron which will be manifested as an electrical impulse indicative of the event. Moreover, certain of the aforenoted processes may work in seriatim yieldingelectrons which, as they are being swept from the detector will subsequently interact to produce photons which may continue along with or without further interaction. For example, a scattered gamma ray following a Compton event may itself interact within the detector. Following a pair-production event, the positron may annihilate with an electron producing two quanta of 511 keV; one or both of these photons may then interact within the detector. Whatever the order of events, if all of the energy of the incident gamma ray is deposited in the active volume of the detector, a signal will be obtained which contributes to the full energy peak or "photopeak" in the pulse-height spectrum. The size and geometry of the detector, as well as the energy of the incident gamma ray, as effected by the electric field within the detector, play an important role in determining (1) the number of events which appear in the full-energy peak for a given incident flux, and (2) the energy resolution of the detector.
In the study and analysis of gamma or x-ray radiation from various sources, one finds gamma rays of various energies and intensities. Because the resulting pulse spectrum from a single gamma ray is itself complex, the resulting spectrum from a source of many gamma rays is even more complex. Thus, the analysis of a spectrum of gamma rays is often difficult and ambiguous with numerous competing interactions giving rise to undesirable background as well as a loss of much useful information should the detector inherently lack sufficient energy resolution.
Obviously, if the detector lacks resolution, for any reason, for portions of the incident spectrum or secondary interactions, associated analytical equipment will not generally be capable of remedial correction in the first instance. Therefore, it is manifestly important that the semiconductor detector possesses great resolution to photon energy without itself contributing to excessive noise. As the resolution and efficiency of these devices are dependent both upon the volume of the active, intrinsic region and the biasing voltage therefor, it is desirable to have each of these factors as large as possible. If the application of a high bias voltage causes excessive leakage current, however, the resolution will suffer.
With regard to the volume of the active region, the prior art is replete with methods and devices to enlarge the active region. The initial step forward was provided by Pell, U.S. Pat. No. 2,957,789 disclosing the lithium drifting of semiconductor material. This technique has been perfected in the prior art to yield P-I-N structures having appropriately large intrinsic zones.
The drifting of lithium in, for example, germanium detectors is now a well known technique to compensate for impurities to yield an n-type region. Generally, lithium drifting techniques are described in U.S. Pat. Nos. 3,225,198, 3,378,414, 3,329,538, 3,472,711, 3,310,443, and No. 3,374,124.
While the prior art has been successful in the fabrication of P-I-N structures having adequately large intrinsic zones and capable of tolerating large biasing voltages without excessive or runaway leakage currents, it has not yet been successful in achieving these results in P-N structures using high-purity germanium instead of lithium-drifting.
As a reverse bias voltage is applied to a P-N detector made from high-purity germanium a depletion zone resulting from the sweeping away of charge carriers will extend partially into the n-type region and partially into the p-type region. The extension of the depletion zone will be primarily into the region of higher purity, which for purposes of the discussion herein we assume to be the p-type region. As is usual in semiconductor devices, the roles of p-type and n-type regions may be interchanged with suitable alterations in the polarities of the charge carriers and the applied voltages. As the reverse bias voltage is increased the thickness of the depletion zone will increase until the surface of the zone becomes contiguous with the external surface of the detector. In a planar-geometry detector this surface is often called the rear surface, although in practice it is customary to orient the detector so that the so-called rear surface actually faces the incident flux of radiation which is to be measured. In a coaxial-geometry detector, depletion generally begins at the outer cylindrical surface, and the depletion zone will at some reverse bias voltage reach the inner surface of the detector. For both planar and coaxial detectors the voltage which is required to deplete the detector throughout to the external surface as hereabove described, is termed the "full depletion voltage" and the condition existing in the detector under the effect of reverse bias equal to or greater than the full depletion voltage is termed "full depletion."
It is important to the successful performance of coaxial detectors made from high purity germanium that such detectors be operated with a reverse bias voltage that is substantially higher than the full depletion voltage. If they are not so operated the electrical field in the region proximate to the external surface at the boundary of the depletion zone is quite weak and the electrical carriers which are generated in this region by the interaction of photons with the detector will be poorly collected and the energy resolution from such events will be poor.
In order to permit the application of a reverse bias voltage higher than the full depletion voltage the usual practice in the prior art has been to apply to the external surface a thin coating of a metal having a sufficiently high work function to hinder or prevent the injection of electrical carriers into the detector at the point of electrical contact. In this manner some success has been achieved in applying a high reverse bias voltage without the undersirable effect of creating excessively high leakage currents which can cause electronic noise and degrade the energy resolution of the detector. To date, however, no totally effective technique has been devised.
The deficiencies in making satisfactory electrical contact with the detector have further ramifications. Any scratches in the metallic surface at the point of contact will provide a preferential conduction path and concomitant breakdown of electrical characteristics via injection of charge carriers. Even if it is possible to affix the contact to the detector in a scratch-free manner, mere vibration resulting from the operation of fans and the like in associated equipment is often sufficient to cause subsequent degradation at the point of contact.
Accordingly, the need exists to provide a radiation detector of high efficiency. Similarly, the need exists to provide such a detector capable of withstanding much higher reverse bias voltages than heretofore obtainable without resulting in breakdown or other deleterious manifestations by injection of charge carriers.